The invention relates to a chemical processing system that uses a vaporized liquid or solid source material. More particularly, the invention relates to a vapor deposition system that uses a vaporized liquid or solid source material.
Liquid or solid source materials are used in many chemical processing systems, such as, for example, Chemical Vapor Deposition (CVD) processes. The liquid or solid source material is typically vaporized in a source container. In a CVD process, the vapor is fed to a reaction chamber in which the vaporized source material is subjected to a chemical reaction and a film is deposited onto a substrate. To ensure that an adequate and constant amount of vapor is delivered to the reaction chamber, it is desirable to monitor the vaporization process in the source container.
U.S. Pat. No. 4,436,674 discloses a vapor mass flow control system wherein a controlled amount of carrier gas is bubbled through a liquid source material in a source container of known temperature and pressure and the level of the liquid source material is sensed. In this system, it is assumed that under constant conditions a constant degree of saturation of the carrier gas with reactant vapor is achieved. As such, when the level of the reactant in the source container decreases, the degree of saturation will decrease. It is therefore important to be able to sense the level of the source material and to control the level of source material within a certain range.
In case of highly reactive source materials, such as those as used in Metal Organic CVD (MOCVD) or Atomic Layer Deposition (ALD), sensing the level of the reactant may be difficult because the source container is made of a robust metal. In addition, it is generally not desirable to place level sensing devices inside the source container as they may be damaged by the reactive source materials. Further, in the case of solid source materials, it is particularly difficult to sense the level or amount of source material in the source container.
A method to measure the amount of reactant in a source container without a level sensor is disclosed by U.S. Pat. No. 6,038,919 to Schmitt et al. In the method disclosed by Schmitt et al, a source container with a known volume is isolated. A known amount of inert gas is then fed into the source container. The temperature of the source container is monitored while the pressure rise due to the gas supply is measured. The free volume of the gas in the source container is calculated using the gas law of Boyle-Gay-Lussac. By subtracting the free volume of gas from the total inner volume of the source container, the volume of the solid or liquid material can be determined.
However, the method of Schmitt et al. has several disadvantages. For example, solid source materials have a tendency to develop a crust on the outer surface that hampers vaporization. That is, although sufficient material might be present in the source container and detected by the Schmitt et al. procedure, the condition of the material is such that not enough vapor is produced. Also in the case of liquid source material, contamination might float on the top surface of the liquid, which can also hamper the vaporization.
Therefore, there is not only a need to measure the amount of liquid or solid source material but also a need to monitor the capability of the source material in the source container to deliver vaporized source material.
Accordingly, one aspect of the present invention is a method to monitor the capability of a source container comprising liquid or solid source material to produce vaporized source material. In one embodiment, the method comprises the steps of: extracting vaporized source material from the source container, thereby reducing the amount of vaporized source material in the source container, isolating the source container; measuring a property that is indicative of a partial pressure of the vaporized source material in the source container as a function of time; comparing the measured property as a function of time with a reference property as a function of time; and generating a signal when the difference between the measured property as a function of time and the reference property as a function of time is larger than a predetermined value.